Embodiments of the invention described herein relate to a method and device for producing electricity by conversion of the mechanical energy of wind or other moving fluids.
Wind power, one of the most promising sources of renewable energy, is starting to be adopted more and more globally. Conventional wind power is based on wind turbines. Conventional wind turbines are rotating machines that convert the kinetic energy in wind into mechanical energy. Wind turbines can be separated into two types based by the axis around which the turbine rotates. Most turbines rotate around a horizontal axis, but some designs have been proposed where the turbine rotates around a vertical axis. Globally, the installed wind capacity was in excess of 120 GW by the end of 2008.
Horizontal axis wind turbines used in commercial wind farms are usually of a three-bladed design. Computer controlled motors orient the blades to face the wind direction. A gear box is commonly used to step up the speed of the generator, although designs may also use direct drive of an annular generator. Some models operate at constant speed, but more energy can be collected by variable-speed turbines which use a solid-state power converter to interface to the transmission system. Most turbines are equipped with shut-down features to avoid damage at high wind speeds.
While wind power adoption has been increasing, there are still issues to be overcome before it can be cost competitive with conventional power on its own merit, without government subsidies or tax credits. One of the issues is the high cost associated with tower height. Due to the inability to harvest sufficient energy at low wind-speeds (i.e., low heights) using conventional designs, the towers need to be very tall as wind speeds are greater at higher altitude. Unfortunately, this requirement can increase the cost of a wind turbine substantially. Transportation of the tall towers and blades can account for up to 20% of the total installed cost of a wind farm. Massive tower construction is required to support the heavy blades, gearbox, and generator. Furthermore, these tall turbines require expensive cranes and skilled operators. Increased tower height also increases maintenance costs as cyclic stress, fatigue and vibration tend to cause failure more frequently in taller turbines. Tower height can even increase public relations costs as taller turbines are likely to increase complaints from residents about damage to their landscape views. Generator designs that allow for a greater conversion efficiency allowing for a similar power rating to be achieved at a smaller tower height/wind speed, therefore, have a commercial advantage.
Another problem with conventional wind-turbines is the technology's low capacity factor. Capacity factor is the ratio of the actual amount of power produced by the wind turbine over time relative to the power that would have been produced if turbine operated at maximum output 100% of the time. The typical capacity factor of a conventional wind turbine is 25-40% as the wind turbine is designed to work only between specific wind-speeds. At low wind-speeds, below the “cut in” speed, the turbine blades do not rotate. At very high wind-speeds they are designed to stop operating for safety reasons. The idle time results in an effectively high cost of energy, and a resulting problem with these types of turbines is that they are typically not used as primary power sources due to the unreliability of the power output. Wind turbine designs that allow for operation across a wider range of wind speeds, which will increase the capacity factor, are therefore beneficial.
One feature of conventional generators that results in a relatively high cut-in speed requirement before significant power is generated by the turbine is the fact that these turbines need a gearbox to convert the low revolutions per minute (typically 0-60 RPM) rotations of the rotor to high RPM rotations of a generator shaft (typically over 1000 RPM). The gearbox results in mechanical energy loss, unacceptable component failure rates, and a relatively high cut-in speed requirement.
Finally, smaller generator designs that have smaller size and weight relative to conventional wind turbines are advantageous because they have lower capital, transportation, installation, and maintenance costs than heavier/bigger generators.